This invention is related generally to an electrophotographic process and apparatus, and is more particularly concerned with improvements in an electrophotographic process of the type in which an electrostatic latent image is formed in conformity with a pattern on an original to be copied subsequent to the sequential steps of neutralizing any residual charges on the surface of an endless photosensitive member by using an eraser lamp, charging the surface of the endless photosensitive member, and projecting an optical image onto the charged surface of the photosensitive member in conformity with the pattern on the original.
Recently, there have been developed in the field of electrophtography, processes wherein electrophotographic copying machines use a latent image transfer system. An electrostatic latent image in conformity with a pattern on an original to be copied is formed on the surface of a photosensitive member, after which, the electrostatic latent image is transferred onto a sheet of copy paper. The copy paper is then developed to make the image thereon visible.
In general, a latent-image-transfer type of copying machine using an electrostatic latent image transfer system dispenses with a cleaning device for removing residual toner which is clinging to the surface of the photosensitive member in contrast to a copying machine adopting a developed toner image transfer system. Therefore, the former system permits the use of an endless photosensitive member of a drum type having a small diameter, e.g., on the order of 60 mm, and presents a simple structure and easy maintenance. In addition, the former system further provides an advantage in that the copy paper to be used may be of a type which is similar to a plain paper yet producing a copy of high quality in contrast to a copying machine embodying an Electrofax type of system. The latent-image-transfer type of copying machine retains all of the aforesaid numerous advantages and, thus has achieved an excellent reputation in the field of electrophotographic copying machines.
However, in practical applications of the electrostatic-latent-image-transfer type of copying machine, a problem is posed. Particularly in those instances where an endless photosensitive member of the drum type having a diameter of about 60 mm is used, a stained copied pattern or image results. The character of the stain depends on the specific photosensitive materials and processes for manufacturing those photosensitive materials which are employed. The staining results in an accompanying lowering in quality of the copy obtained.
This will be described in more detail hereinbelow. In the case of an endless photosensitive member of the drum type having a diameter of about 60 mm as is used commonly in an electrostatic-latent-image-tranfer copying machine, the outer circumference of the drum is about 190 mm. Therefore, the drum photosensitive member must be rotated at least one full turn to complete the copying of a sheet of an original so that often a portion of the surface of the drum photosensitive member must be used repeatedly. More particularly, during the copying of a sheet of A4 size, (the length of an A4-size sheet is 297 mm) about 60% of the peripheral surface of the drum photosensitive member must be used twice. Accordingly, the first portion of a copy produced by the machine (the portion of a copied pattern corresponding an electrostatic latent image formed on the photosensitive member during the first rotational cycle of the drum photosensitive member) is subject to fogging. (This will be referred to hereinafter as a foggy effect.) In addition, a later portion of the copy (the copied pattern corresponding to an electrostatic latent image formed on the photosensitive member during the second cycle of rotation of the drum photosensitive member, i.e., a latent image formed on the repeatedly used surface of the photosensitive member) exhibits a superimposed copied pattern of that of the beginning half of a copy. (This will be referred to hereinafter as a "memory" effect.) Still in addition, stains of a mesh form appear in the middle portion of the copy.